This invention relates generally to medical imaging. More particularly, this invention relates to spiral scanning for medical imaging.
Medical imaging, for example Magnetic Resonance Imaging (MRI), involves acquiring data in the spatial frequency domain referred to as k-space, and transforming the data into the spatial domain prior to viewing. The k-space is represented by magnitude and phase time data. The Fourier transform of k-space data is the MRI image.
Spiral scanning is a known method in dynamic imaging, for example MRI, that has been used to achieve shorter scan time than can be achieved with conventional MR techniques, such as raster scanning. The method of a spiral imaging sequence, or spiral acquisition, involves acquiring data from k-space in a spiral. In a spiral acquisition, the gradients in the logical X- and Y-axes start from zero, and increase in amplitude in a quasi-periodic fashion. This has the effect of a trajectory that starts from the center of k-space and spirals out to some maximum value. Spiral acquisitions are used in dynamic imaging because they cover k-space more efficiently than the simpler raster methods.
A spiral of constant pitch, also known as an Archimedian spiral, is a known technique for 2D selective excitations limited by both the gradient amplitude and gradient slew rate. The design of spiral acquisitions involves traversing the spiral angle at the ⅔ power of time for constant slew rate and the xc2xd power of time for constant gradient amplitude. However, near the origin of k-space these analytic relations do not hold. There is a singularity near the origin of k-space, which can be solved by numerical integration of the trajectory differential equations.
The numerical method is computationally demanding and complex which results in longer scan time. The effect of time delays directly affects the patient or subject being diagnosed by the imaging device. Any change of a parameter affecting the gradient waveforms on the imaging device requires that the numerical integration be recalculated, and further requires the patient remain in position in the medical imaging device. What is needed is a computationally less demanding and simpler spiral trajectory calculation that does not compromise the resolution of the resultant medical image. There is a further need to reduce the calculation time for a spiral scanning sequence, thereby reducing the time a patient spends in a medical imaging device.
A computationally efficient method for calculating a spiral scanning trajectory for magnetic resonance imaging (MRI) comprises the steps of placing an object to be imaged in a magnetic field, exciting nuclei in the object to excite desired spectral components for imaging and applying time-varying gradients along two axes in accordance with at least one spiral trajectory in k-space. The method includes generating the spiral trajectory in k-space by analytical approximation. A set of sampling point locations are defined along the spiral trajectory. Data samples are acquired at the sample point locations and processed to construct an image of the object.
An apparatus for MRI by spiral trajectory scanning comprises an MR imaging device for acquiring and processing data samples of an object to be imaged, wherein the data samples are acquired by spiral trajectory scanning. An interface is coupled to the MR imaging device for receiving operator-defined spiral scanning parameters, and a computer is coupled to the interface and the MR imaging device. The computer is configured to generate an approximated spiral trajectory in k-space in accordance with operator-defined spiral scanning parameters. The approximated spiral trajectory is used by the MR imaging system to perform spiral trajectory scanning.